The invention relates to radio navigation on the basis of the propagation time of an electromagnetic wave with a time marker between a transmitter and a receiver.
In recent navigation systems, the time marker of the carrier wave is of the repetitive pseudo-noise type; in practice, a random noise code is used. On reception, the propagation time of the wave is demonstrated both by a time off-set in the pseudo-random code and by a phase off-set in the carrier wave. In the context of relative movement on the transmitter/receiver line (line of sight), there is added a carrier frequency off-set due to the Doppler effect.
There are various sources of error: Some of them in connection with passage through the troposphere or ionosphere or variation in conductivity of ground surfaces--will affect the speed of propagation of waves in general; other error sources take account of the fact that the path followed by the radioelectrical wave as far as the receiver is not rectilinear, whilst others arise from the fact that the receiver experiences a combination of different paths originating from various reflections, which are generally due to surfaces adjacent to the reception antenna. These are termed multiple path errors: the direct path (the shortest) is overlaid with other, unwanted paths.
Within radio navigation systems, the most frequently employed systems at present, "GPS" and "GLONASS", their transmissions originate from satellites.
Currently, the main problem is how to combat the effects of multiple paths, as instanced by the article "Conquering multipath: The GPS accuracy battle", by Lawrence R WEILL, in the revue GPS World, April 1997, pages 59-66.